Ideal biomarkers of acute and delayed radiation injury after a radiological/nuclear terrorist incident are those that arise and are measurable prio to manifestation of tissue injuries, typically one to a few days after ionizing radiation (IR) exposure. They should also be measurable in a non-invasive or minimally invasive way - for example, using peripheral blood samples. High-dose IR induces acute and delayed injuries to both hematopoietic and solid tissues. Hematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs) are critical for regenerating and repairing these tissues. MSCs differentiate into osteoblasts, chondrocytes, and adipocytes while HSCs are precursors of various types of blood cells. We showed that HSCs and MSCs undergo IR-induced senescence, a stable post-mitotic state, after IR. In contrast, most differentiated blood cells undergo apoptosis after IR and are quickly cleared. HSCs and MSCs generally reside in the bone marrow, but small numbers can always be detected and isolated from peripheral blood. In this SBIR project, we propose to use cellular senescence as the biological end-point for radiation biodosimetry. We will develop integrated microfluidic chips, termed Senescence-Chips, for rapid and accurate detection of senescent cells, particularly, IR-induced senescent HSCs and MSCs, as well as circulating cytokines/chemokines due to the senescence-associated secretory phenotype (SASP), from small volumes of human peripheral blood. The project builds on our recently published progress in characterizing the IR-induced senescence of HSCs and MSCs, the SASP as a stable (chronic) phenotype of senescent cells, and the development of a new mouse model (p16-3MR C57BL/6 mice) that allows us to identify, track - and, importantly, inducibly kill - senescent cell in vivo and at will. Our novel microfluidic chips contain multiple functional modules that will capture and enumerate the total and senescent populations of HSCs and MSCs, down to single cells, and simultaneously detect SASP, in peripheral blood. The chips will be validated using cell lines, mouse models, and human clinical samples. Senescence-chips will enable low-cost, reproducible, highly specific and sensitive multiplex measurements of human peripheral blood for radiation biodosimetry, thus serving as a field-deployable platform for radiological/nuclear medical countermeasures including emergency triage and medical responses.